At the present time, there is increasing interest and use of composite materials. Typically, such composite materials comprise various layers or plys of a fiber, e.g. glass fiber, carbon fiber, Kevlar (trade mark) or some other fiber, bonded together by a resin or polymer. The fibers provide the strength to the material, and with fibers being developed, composite materials can be extremely strong and light weight, i.e. with a good strength-to-weight ratio. Further, composite materials can have a tailored stiffness, can readily be molded into complex shapes, and have good thermal expansion properties and resistance to fatigue and corrosion.
For these various reasons, composite materials are gaining wide acceptance in many industries, notably the aircraft industry. It will be appreciated that, in an aircraft, the integrity of structural components is vital. At the same time, because of weight considerations, one does not want to build in any excessive redundancy into the aircraft structure. Accordingly, it is necessary to monitor components formed from reinforced composite materials.
Unfortunately, this is made difficult by the fact that composite materials have some unique failure modes. Their very nature endows them with anisotropic properties, and in particular, the polymer or resin matrix is at least and order of magnitude weaker than the embedded fibers of glass, carbon or Kevlar. As a consequence, failure of the matrix can occur, without any damage to individual fibers. Cracking parallel to material fibers within a given ply and disbonding between successive plys can occur, such cracking being known as "delamination". Unfortunately, a relatively low energy impact, e.g. a dropped tool, or a high energy impact with a bird in flight, can cause an area of delamination. Once started, the region of damage or delamination can grow with continued stress cycling.
A further problem with such damage is that it is often not evident from external visual inspection. It will be appreciated that, of necessity, where a component is in service nondestructive techniques must be used. Present nondestructive diagnostic techniques include X-rays; ultrasonic scanning systems; holography; vibrothermography; acoustic emission analysis; and structural monitoring using optical fibers. At least for X-ray examination and ultrasound C-scan examination, a considerable amount of time and experience has been invested in developing these techniques. However, they have a number of limitations. Further, there are numerous situations, where they cannot be used. Thus, for an aircraft leading edge which has a rubber or resilient de-icing boot, an ultrasound examination technique cannot be used, unless this is removed. Many other techniques, apart from those using embedded optical fibers, require dismantling of the structure. Consequently, they are time consuming and expensive, and often require a skilled technician to perform the test.
Monitoring techniques have been proposed, using a network of optical fibers embedded into the structure of a component and forming an integral part of it, and such techniques potentially have a number of advantages. Theoretically, through the use of optical fiber sensing technology can be used for a number of purposes: e.g. impact detection and location; delamination and microcrack detection and location; strain and deformation mapping. Structurally embedded optical fibers have the advantage of being light weight and small. They require no electromagnetic shielding, provide no conductive path and possess a high bandwidth which can avoid the use of multiple cables. They are insensitive to electromagnetic interference, are inert and are safe. Further, the optical fibers could be used to monitor a component from initial manufacture to the end of its useful life. Thus, there can be used to provide information on the state of a structural component throughout manufacture, installation, maintenance and use. During manufacture, they could be used to provide information on the degree of cure, and hence are for the possibility of better quality control. Carefully positioned optical fibers can be used to provide a warning of incorrect installation, e.g. by being fitted around rivet holes. During use or maintenance, the optical fiber sensing can monitor any damage inflicted to the structure.
It is known to embed optical fibers into a composite material, for detecting damage. One proposal can be found in U.S. Pat. No. 4,581,527 (Crane et al). This discloses a damage assessment system using a three-dimensional grid of optical fibers. The fibers are located in a number of layers. Like many earlier proposals, this patent suggests that it is necessary to use an orthogonal grid of optical fibers to determine a damaged area. Further, it apparently suggest that, even for a panel, it is necessary to provide a grid of optical fibers at different depths in order to detect damage. There is no discussion as to how the composite material fails, or the relationship between the strength and failure of the optical fibers, and the failure of the composite material.
U.S. Pat. Nos. 4,603,252 and 4,629,318 to Malek and Hofer, and also an article by Bernd Hofer in the September 1987 issue of Composites, vol. 18, pages 309-316, describe other fiber optic damage detection systems. U.S. Pat. No. 4,603,252 describes the technique in which optical fibers are again laid in an orthogonal grid pattern. The patent suggests that individual optical fibers are used to replace some of the carbon reinforcing fibers. It is not clear how this would be achieved, since in practice, the reinforcing fibers are usually provided as a "prepreg" comprising a mesh of fibers impregnated with a resin. As the optical fibers replace the reinforcing fibers, they necessarily are collinear with the reinforcing fibers and this is clearly shown. The patent also mentions the possibility of providing surface roughness on the fibers, to increase their adhesion to the surrounding synthetic material. No precise indication is given as to size of the microscopic notches relative to the size of the optical fiber. Further, it is suggested that this roughness could be provided by etching; since etching conventionally smooths a surface, it is not clear how this would be achieved. Again, there is no discussion as to how a composite material fails, nor any discussion as to how the optical fibers should be laid relative to the reinforcing fibers, to achieve the most reliable performance. The second Malek and Hofer U.S. Pat. No. 4,629,318 provides a measuring device for determining cracks in a test object, which provides an arrangement in which a number of optical fibers can be scanned sequentially, to determine which fibers have failed. A number of different orientations of fibers, e.g. around rivet holes, is suggested. The patent also teaches a technique in which the optical fibers are mounted on an adhesive foil, for mounting on the surface of a structure. However, there is no teaching of a specific method of incorporating the optical fibers into a composite material. The basic technique relies upon scanning light across interruptions in the optical fibers, with the interruptions being aligned with one another. One then checks for the emission of light at either end of each fiber. If there is no light emission, then this is indicative of a rupture in the optical fiber, and hence of a failure in the object to which it is bonded.
The article by Hofer in the September 1987 of Composites describes a system of optical fibers integrated into a composite structure, and includes examples of applications of this technique to aircraft and other components. Whilst some of the described techniques use optical fibers extending in just one direction, there is no teaching as to how such uni-directional fibers can be used to define the entire boundary of a damaged area. Here, Hofer only teaches that the absence of light transmission through a fiber indicates a damaged area along its length. Again, mention is made of a chemical treatment for the surface of the fibers, with the intention of influencing the strength of the fibers. Whilst this article recognizes that impacts on composite panels cause damage at the rear side of the panel, there is no appreciation of the details of the delamination mechanism.
Finally, U.S. Pat. No. 4,772,092, again to Hofer and Malek, describes a crack detection arrangement using optical fibers as reinforcement fibers. Here, light conducting fibers are substituted for regular reinforcing fibers in a multiple lamina, with the optical fibers being woven in with the reinforcing fibers. No details are given as to how such an arrangement would be employed in use.